No. It would be nice though. There's a strong opinion that it shouldn't be an easy option like this, if a member wants to delete their content they should have to do it piece by piece.
Members should be backing up their content, there are many ways. And you can archive your network, Ning provides software (but not space) for that.
But don't let members jerk you around on this. A deliberate choice like that should be permanent. You can't spend hours restoring content to someone who clearly acts on whim and does not respect your time....
That's my opinion!
For me it's less about the member wanting their content back (that's their own fault) but more about the other members' content that gets deleted. Suppose that member had started many discussions - everyone's replies are now gone too.